1. Field of the Invention
The present invention relates to an air-fuel ratio detection apparatus for detecting, over a wide range, the air-fuel ratio of exhaust gas discharged from combustion equipment such as an internal combustion engine. More particularly, the present invention relates to an air-fuel ratio detection apparatus which enables accurate and fast air-fuel-ratio feedback control of combustion equipment after startup of the air-fuel ratio detection apparatus.
2. Description of the Related Art
For air-fuel ratio control of an internal combustion engine, an air-fuel ratio detection apparatus is known including a gas sensor for detecting the air-fuel ratio of exhaust gas discharged from an internal combustion engine. Known gas sensors for such a purpose include a sensor (λ sensor) that outputs one of two levels in accordance with the concentration of oxygen in exhaust gas (in accordance with whether the air-fuel ratio is rich or lean), and a sensor (called a full range air-fuel-ratio sensor, a linear air-fuel-ratio sensor, or the like; hereinafter also referred to as a “linear sensor”) that outputs a sensing output over a wide oxygen concentration (air-fuel ratio) range while maintaining linearity. In recent years, in order to cope with strengthened emission controls which require a reduction in the emission of hazardous gas, demands have arisen to control the air-fuel ratio of an air-fuel mixture supplied to an internal combustion engine over a wide range. In view of this, a technique has been embodied in which a linear sensor is employed in place of a λ sensor, and air-fuel-ratio feedback control is performed on the basis of an output from the linear sensor.
Incidentally, a gas sensor of any of the above-described types mainly employs a structure in which a pair of electrodes are provided on opposite surfaces of a solid electrolyte to thereby form a cell. The gas sensor detects oxygen concentration (air-fuel ratio) by an electromotive force generated due to a difference in oxygen concentration between atmospheres to which opposite surfaces of the solid electrolyte are exposed, or by movement of oxygen ions via the solid electrolyte when current flows between the electrodes. These phenomena do not occur unless the solid electrolyte is heated to a certain temperature or higher and brought into a so-called active state. Therefore, consideration has been given to providing a heater in an air-fuel ratio detection apparatus so as to heat a gas sensor in order to quickly activate the same, and to thereby rapidly perform air-fuel-ratio feedback control on the basis of the output of the gas sensor after startup of an internal combustion engine.
However, a linear sensor having recently been put into practice requires a very long time of ten seconds to several tens of seconds before its cell is sufficiently activated and the sensor generates a stable output as a linear sensor, even when the linear sensor is heated by means of a heater. In order to overcome this drawback, an air-fuel-ratio detection apparatus as disclosed in Patent Documents 1 and 2 has been proposed in which such a linear sensor is used, and which enables air-fuel-ratio feedback control to be quickly performed after startup of an internal combustion engine.
Patent Documents 1 and 2 disclose a technique for determining whether a linear sensor has entered a semi-activated state. A determination as to whether the air-fuel ratio is a rich-side air-fuel ratio or a lean-side air-fuel ratio can be performed on the basis of the sensor output, in a stage before the linear sensor has entered a fully activated state (completely activated state) after startup of the internal combustion engine. In this state, the sensor outputs a linear sensing output in accordance with the air-fuel ratio. These publications state that determination as to whether the air-fuel ratio is a rich-side air-fuel ratio or a lean-side air-fuel ratio is performed on the basis of a sensing output at the time the cell of the linear sensor is determined to have reached the semi-activated state.
[Patent Document 1] Japanese Patent Application Laid-Open (kokai) No. H9-170997
[Patent Document 2] Japanese Patent Application Laid-Open (kokai) No. 2004-69547
3. Problems To Be Solved By The Invention
From a different aspect, in addition to a limit-current-type sensor having a single cell disclosed in Patent Document 2, a layered-type gas sensor in which a pump cell and an oxygen-concentration measurement cell are layered is known as a linear sensor capable of detecting air-fuel ratio (oxygen concentration) over a wide range. More specifically, this layered-type gas sensor is configured such that pump and oxygen-concentration measurement cells each composed of a solid electrolytic layer and a pair of electrodes sandwiching the solid electrolytic layer are integrally layered. In this arrangement, one electrode of each cell is exposed to a measurement gas chamber, into which exhaust gas can be introduced via a diffusion control section.
For such a linear sensor composed of a plurality of cells, a technique for quickly starting air-fuel-ratio feedback control after startup of an internal combustion engine has been studied. In this technique, determination as to whether the air-fuel ratio is a rich-side air-fuel ratio or a lean-side air-fuel ratio is performed by use of an output of the sensor in a stage before the sensor reaches the fully activated state. A specific example thereof is a sensor control apparatus proposed in Japanese Patent Application No. 2005-264879, which had not yet been laid-open at the time of filing of this application. The sensor control apparatus determines whether a gas sensor (linear sensor) including a plurality of cells has reached a semi-activated state in a stage before the gas sensor reaches a fully activated state. After determining that the gas sensor cells have reached the semi-activated state, the sensor control apparatus determines whether the air-fuel ratio is on the rich side or the lean side on the basis of a voltage generated between the electrodes of one of the cells (e.g., an oxygen concentration measurement cell).
However, through keen studies, the present inventors found that, after the gas sensor including a plurality of cells has reached a semi-activated state, the voltage generated between the electrodes of the oxygen concentration measurement cell changes with delay in relation to an actual change in the air-fuel ratio, and the responsiveness of sensing output is not satisfactory. This phenomenon is considered to occur because exhaust gas introduced into the measurement gas chamber must pass through a diffusion control section, and replacement of gas in the measurement gas chamber occurs slowly because of the presence of the diffusion control section. Accordingly, the sensor control apparatus described in the prior application can carryout feedback control from a stage before the gas sensor has entered a fully activated state after startup of the sensor control apparatus. However, in order to perform accurate air-fuel-ratio feedback control in consideration of further rigorous strengthened emission control stands, the sensor control apparatus must have enhanced responsiveness, to changes in air-fuel ratio, of the sensing output generated once the gas sensor has reached the semi-activated state.